The present invention concerns a titanium compressor wheel for use in an air boost device, capable of operating at high RPM with acceptable aerodynamic performance, yet capable of being produced economically by an investment casting process.
Air boost devices (turbochargers, superchargers, electric compressors, etc.) are used to increase combustion air throughput and density, thereby increasing power and responsiveness of internal combustion engines. The design and function of turbochargers are described in detail in the prior art, for example, U.S. Pat. Nos. 4,705,463, 5,399,064, and 6,164,931, the disclosures of which are incorporated herein by reference.
The blades of a compressor wheel have a highly complex shape, for (a) drawing air in axially, (b) accelerating it centrifugally, and (c) discharging air radially outward at elevated pressure into the volute-shaped chamber of a compressor housing. In order to accomplish these three distinct functions with maximum efficiently and minimum turbulence, the blades can be said to have three separate regions.
First, the leading edge of the blade can be described as a sharp pitch helix, adapted for scooping air in and moving air axially. Considering only the leading edge of the blade, the cantilevered or outboard tip travels faster (MPS) than the part closest to the hub, and is generally provided with an even greater pitch angle than the part closest to the hub (see FIG. 1). Thus, the angle of attack of the leading edge of the blade undergoes a twist from lower pitch near the hub to a higher pitch at the outer tip of the leading edge. Further, the leading edge of the blade generally is bowed, and is not planar. Further yet, the leading edge of the blade generally has a xe2x80x9cdipxe2x80x9d near the hub and a xe2x80x9crisexe2x80x9d or convexity along the outer third of the blade tip. These design features are all designed to enhance the function of drawing air in axially.
Next, in the second region of the blades, the blades are curved in a manner to change the direction of the airflow from axial to radial, and at the same time to rapidly spin the air centrifugally and accelerate the air to a high velocity, so that when diffused in a volute chamber after leaving the impeller the energy is recovered in the form of increased pressure. Air is trapped in airflow channels defined between the blades, as well as between the inner wall of the compressor wheel housing and the radially enlarged disc-like portion of the hub which defines a floor space, the housing-floor spacing narrowing in the direction of air flow.
Finally, in the third region, the blades terminate in a trailing edge, which is designed for propelling air radially out of the compressor wheel. The design of this blade trailing edge is generally complex, provided with (a) a pitch, (b) an angle offset from radial, and/or (c) a back taper or back sweep (which, together with the forward sweep at the leading edge, provides the blade with an overall xe2x80x9cSxe2x80x9d shape). Air expelled in this way has not only high flow, but also high pressure.
Recently, tighter regulation of engine exhaust emissions has led to an interest in even higher pressure ratio boosting devices. However, current compressor wheels are not capable of withstanding repeated exposure to higher pressure ratios ( greater than 3.8). While aluminum is a material of choice for compressor wheels due to low weight and low cost, the temperature at the blade tips, and the stresses due to increased centrifugal forces at high RPM, exceed the capability of conventionally employed aluminum alloys. Refinements have been made to aluminum compressor wheels, but due to the inherent limited strength of aluminum, no further significant improvements can be expected. Accordingly, high pressure ratio boost devices have been found in practice to have short life, to be associated with high maintenance cost, and thus have too high a product life cost for widespread acceptance.
Titanium, known for high strength and low weight, might at first seem to be a suitable next generation material. Large titanium compressor wheels have in fact long been used in turbojet engines and jet engines from the B-52B/RB-52B to the F-22. However, titanium is one of the most difficult metals to work with, and currently the cost of production associated with titanium compressor wheels is so high as to limit wide spread employment of titanium.
There are presently no known cost-effective manufacturing techniques for manufacturing automobile or truck industry scale titanium compressor wheels. The automotive industry is driven by economics. While there is a need for a high performance compressor wheel, it must be capable of being manufactured at reasonable cost.
One example of a patent teaching casting of compressor wheels is U.S. Pat. No. 4,556,528 (Gersch et al) entitled xe2x80x9cMethod and Device for Casting of Fragile and Complex Shapesxe2x80x9d. This patent illustrates the complex design of compressor wheels (as discussed in detail above), and the complex process involved in forming a resilient pattern for subsequent use in forming molds. More specifically, Gersch et al teach a process involving placing a solid positive resilient master pattern of an impeller into a suitable flask, pouring a flexible and resilient material, such as silastic or platinum rubber material, over the master pattern, curing, and withdrawing the solid master pattern of the impeller from the flexible material to form a flexible mold with a reverse or negative cavity of the master pattern. A flexible and resilient curable material is then poured into the cavity of the reverse mold. After the flexible and resilient material cures to form a positive flexible pattern of the impeller, it is removed from the flexible negative mold. The flexible positive pattern is then placed in an open top metal flask, and foundry plaster is poured into the flask. After the plaster has set up, the positive flexible pattern is removed from the plaster, leaving a negative plaster mold. A non-ferrous molten material (e.g., aluminum) is poured into the plaster mold. After the non-ferrous molten material solidifies and cools, the plaster is destroyed and removed to produce a positive non-ferrous reproduction of the original part.
While the Gersch et al process is effective for forming cast aluminum compressor wheels, it is limited to non-ferrous or lower temperature or minimally reactive casting materials and cannot be used for producing parts of high temperature casting materials such as ferrous metals and titanium. Titanium, being highly reactive, requires a ceramic shell.
U.S. Pat. No. 6,019,927 (Galliger) entitled xe2x80x9cMethod of Casting a Complex Metal Partxe2x80x9d teaches a method for casting a titanium gas turbine impeller which, though different in shape from a compressor wheel, does have a complex geometry with walls or blades defining undercut spaces. A flexible and resilient positive pattern is made, and the pattern is dipped into a ceramic molding media capable of drying and hardening. The pattern is removed from the media to form a ceramic layer on the flexible pattern, and the layer is coated with sand and air-dried to form a ceramic layer. The dipping, sanding and drying operations are repeated several times to form a multi-layer ceramic shell. The flexible wall pattern is removed from the shell, by partially collapsing with suction if necessary, to form a first ceramic shell mold with a negative cavity defining the part. A second ceramic shell mold is formed on the first shell mold to define the back of the part and a pour passage, and the combined shell molds are fired in a kiln. A high temperature casting material is poured into the shell molds, and after the casting material solidifies, the shell molds are removed by breaking.
It is apparent that the Galliger gas turbine flexible pattern is (a) collapsible and (b) is intended for manufacturing large-dimension gas turbine impellers for jet or turbojet engines. This technique is not suitable for mass-production of automobile scale compressor wheels with thin blades, using a non-collapsing pattern. Galliger does not teach a method which could be adapted to in the automotive industry.
In addition to the above xe2x80x9crubber patternxe2x80x9d technique for forming casting molds, there is a well-known process referred to as xe2x80x9cinvestment castingxe2x80x9d which can be used for making compressor wheels and which involves:
(1) making a wax pattern of a hub with cantilevered airfoils,
(2) casting a refractory mass about the wax pattern,
(3) removing the wax by solvent or thermal means, to form a casting mold,
(4) pouring and solidifying the casting, and
(5) removing the mold materials.
There are however significant problems associated with the initial step of forming the compressor wheel wax pattern. Whenever a die is used to cast the wax pattern, the casting die must be opened to release the product. Herein, the several parts of the die (die inserts) must each be retracted, generally only in a straight (radial) line.
As discussed above, the blades of a compressor wheel have a complex shape. The complex geometry of the compressor wheel, with undercut recesses and/or back tapers created by the twist of the individual air foils with compound curves, not to mention dips and humps along the leading edge of the blade, impedes the withdrawal of die inserts.
In order to side-step these complexities, it has been known to fashion separate molds for each of the wax blades and for the wax hub. The separate wax blades and hub can then be assembled and fused to form a wax compressor wheel pattern. However, it is difficult to assemble a compressor pattern from separate wax parts with the required degree of precisionxe2x80x94including coplanerism of airfoils, proper angle of attack or twist, and equal spacing. Further, stresses are encountered during assembling lead to distortion after removal from the assembly fixture. Finally, this is a labor intensive and thus expensive process. This technique cannot be employed on an industrial scale.
Certainly, titanium compressor wheels would seem desirable over aluminum or steel compressor wheels. Titanium is strong and light-weight, and thus lends itself to producing thin, light-weight compressor wheels which can be driven at high RPM without over-stress due to centrifugal forces.
However, as discussed above, titanium is one of the most difficult materials to work with, resulting in a prohibitively high cost of manufacturing compressor wheels. This manufacturing cost prevents their wide-spread employment. No new technology will be adopted industrially unless accompanied by a cost benefit.
There is thus a need for a simple and economical method for mass producing titanium compressor wheels, and for the low-cost titanium compressor wheels produced thereby. The method must be capable of reliably and reproducibly producing compressor wheels, without suffering from the prior art problems of dimensional or structural imperfections, particularly in the thin blades.
The present invention addressed the problem of whether it would be possible to design a titanium compressor wheel for boosting air pressure and throughput to an internal combustion engine and satisfying the following two (seemingly contradictory) requirements:
aerodynamically: the aerodynamic efficiency, when operating at the high RPM at which titanium compressor wheels are capable of operating, must be comparable to the efficiency of the complex state-of-the-art compressor wheel designs, and
manufacturability: the compressor wheels must be capable of being mass produced in a manner that is more efficient than the conventionally employed methods described above.
The problem was solved by the present inventors in a surprising manner. Simply stated, the present inventors approached this problem by standing it on it""s head. Traditionally, a manufacturing process begins by designing a product, and then devising a processes for making that product. Most compressor wheels are designed for optimum aerodynamic efficiency, and thus have narrow blade spacing and complex leading and trailing edge design (excess rake, undercutting and backsweep, complex bowing and leading edge hump and dip).
The present invention was surprisingly made by departing from the conventional engineering approach and by looking first not at the end product, but rather at the various processes for producing the wax pattern. The inventors then designed various compressor wheels on the basis of xe2x80x9cpullabilityxe2x80x9dxe2x80x94ability to be manufactured using die inserts which are pullablexe2x80x94and then tested the operational properties of various compressor wheels produced from these simplified patterns at high RPM, with repeated load cycles, and for long periods of time (to simulate long use in practical environment). The result was a simplified compressor wheel design which (a) lends itself to economical production by casting of titanium, and (b) at high RPM has an entirely satisfactory aerodynamic performance.
More specifically, the invention provides a titanium compressor wheel with a simplified blade design, which will aerodynamically have a degree of efficiency comparable to that of a complex compressor wheel blade design, aid yet which, form a manufacturing aspect, can be produced economically in an investment casting process (lost wax process) using a wax pattern easily producible at low cost from an automated (and xe2x80x9cpullablexe2x80x9d) die.
As a result of this discovery, the economic equation has shifted for the first time in favor of the titanium compressor wheel for general automotive technology.
Accordingly, in a first embodiment, the invention concerns a compressor wheel of simplified blade design, such that:
a wax pattern can be formed in a die consisting of one or more die inserts per compressor wheel air passage (i.e., the space between the blades), and preferably two die inserts per air passage, and
the die inserts can automatically be extracted radially or along some compound curve or axis in order to expose the wax pattern for easy removal.
The compressor wheel blades may have curvature, and may be of any design so long as the blade leading edges have no dips and no humps, and the blades have no undercut recesses and/or back tapers created by the twist of the individual air foils with compound curves of a magnitude which would prevent extracting the die inserts radially or along some curve or arc in a simple manner.
In simplest form, the wax mold is produced from a die having one die insert corresponding to each air passage. This is possible where the blades are designed to permit pulling of simple die inserts (i.e., one die insert per air passage). However, as discussed below, teach die can be comprised of two or more die inserts of each passage, with two inserts per air passage being preferred for reasons of economy.
In a more advanced form, the blades are designed with some degree of rake or backsweep or curvature, but only to the extent that two or more, preferably two inserts, per air passage can be easily automatically extracted. Such an arrangement, though slightly increasing the cost and complexity of the wax mold tooling, would permit manufacture of wax molds, and thus compressor wheels, with greater complexity of shape. In the case of two inserts per air passage, the pull direction would not necessarily be the same for each member of the pair of inserts. The one die insert, defining one area of the air passage between two blades, may be pulled radially with a slight forward tilt, while a second die insert, defining the rest of the passage, may be pulled along a slight arc due to the slight backsweep of the blade. This embodiment is referred to as a xe2x80x9ccompound die insertxe2x80x9d embodiment. One way of describing pullability is that the blade surfaces are not convex. That is, a positive draft exists along the pull axis.
Once the wax pattern is formed, the titanium investment casting process continues in the conventional manner.
The invention further concerns an economical method for operating an internal combustion engine, comprising providing said engine with an easily manufactured, long-life titanium compressor wheel and driving the titanium compressor wheel at high RPM for increasing combustion air throughput and density and reducing emissions.
The titanium compressor wheel of the present invention has a design lending itself to being produced in a simplified, highly automated process.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood, and so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other compressor wheels for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims.